An interior permanent magnet (IPM) machine is a brushless electric motor having permanent magnets embedded in its rotor core. Permanent magnet electric motors are generally reliable, light, and thermally efficient. In the past, however, permanent magnets have primarily been used on small, low-power electric motors, because of the relative difficulty associated with finding a material capable of retaining a high-strength magnetic field, and rare earth permanent magnet technology being in infancy.
Recent advances in material technology have enabled the creation of lower cost high-intensity permanent magnets. These technological advances have facilitated development of compact, high-power permanent magnet machines for high-volume applications, such as for powering a vehicle, i.e. a hybrid or electric vehicle. IPM machines, in particular, are generally characterized by having favorable ratios of output torque versus the motor's physical size, as well as reduced input voltage. IPM machines are typically reliable, in large part because permanent magnets are retained within dedicated slots of the machine's rotor. When supplied with motive energy from an external source, an IPM machine may also function as a generator. As a result, IPM machines have been gaining acceptance in the transportation industry as viable powerplants for electric and hybrid electric vehicles.